1. Field of the Invention
The present invention relates to phase compensation for providing a transient response characteristic which does not depend on load current of a regulator.
2. Description of the Related Art
FIG. 4 shows a constitution of a conventional regulator. A reference voltage power supply 201 supplies constant voltage Vref to an inverted input terminal of a transconductance amplifier 202. An output of the transconductance amplifier 202 is connected to the gate of a PMOS output driver transistor 204 and a phase compensating RC network 203 constituted by a resistor 208 and a capacitor 209. The source of the PMOS output driver transistor 204 is connected to an input terminal IN and the drain is connected to an output terminal OUT. The output terminal OUT is connected with a load resistor 207 and a capacitor 206 and a voltage dividing circuit 205 constituted by resistors 210 and 211. The voltage dividing circuit 205 supplies voltage produced by dividing output voltage VOUT to a noninverting input terminal of the transconductance amplifier.
When a resistance value of the resistor 208 constituting the phase compensation RC network 203 is designated by notation R208 and a capacitance value of the capacitor 209 is designated by notation C209, frequency fz of a zero point for phase compensation constituted by R208 and C209, is calculated by the following equation.                     fz        =                  1                      2            ⁢                          π              ·              R208              ·              C209                                                          (        1        )            
When a resistance value of the load resistor 207 is designated by notation R207 and a capacitance value of the load capacitor 206 is designated by notation C206, frequency fp of a pole constituted thereby is calculated by the following equation.                     fp        ⁢                  xe2x80x83                =                  xe2x80x83                ⁢                  1                      2            ⁢                          xe2x80x83                        ⁢                          π              ·              R207              ·              C206                                                          (        2        )            
As is apparent from Equation (2), in accordance with a variation in the load resistor 207, the frequency fp of the pole is also changed. Meanwhile, as is apparent from Equation (1), the frequency fz of the zero point for phase compensation is a fixed value.
When load current is large, the load resistor 207 becomes small and accordingly, by Equation (2), the frequency fp of the pole is moved to a high frequency side. Further, when the load current is small, the load resistor 207 becomes large and accordingly, by Equation (2), the frequency fp of the pole is moved to a low frequency side. FIG. 5 shows frequency characteristics of the regulator when the load current is large and when the load current is small.
As shown by FIG. 5, when the load current is large, unity gain frequency at which voltage gain of the regulator becomes 1, becomes high, conversely, when the load current is small, the unity gain frequency becomes low. When the unity gain frequency is changed by the load current in this way, the transient response characteristic depends on the load current, which is not preferable. Particularly, when the load current is small, the unity gain frequency is low and accordingly, the transient response characteristic is deteriorated.
In order to resolve the above-described problem, according to the invention, there is carried out an improvement in which by varying a frequency of a zero point for phase compensation in accordance with load current, a variation in a frequency band of a regulator is restrained such that transient response does not depend upon the load current.
According to the invention, by generating current in proportion to load current by a load current detecting transistor connected in parallel with an output driver transistor for supplying current to a load and changing a resistance value of a variable resistance portion by the current, a frequency of a zero point for phase compensation is varied.
An improvement is carried out by varying the frequency of the zero point for phase compensation in accordance with the load current, thereby, a variation in a frequency band of a regulator is restrained without depending upon the load current such that transient response does not depend upon the load current.